Down Fill Power: Demystifying the Misconceptions

November 18, 2017

If you go shopping for winter jackets or sleeping bags that use down insulation, you will have seen things on the tags that say “650 down fill power” or “800 FP” or maybe just the number “700”. This is a standard unit of measurement for the quality of down. Unfortunately, it is commonly misinterpreted as being the QUANTITY of down rather than the QUALITY. It is understandable why most people interpret it in this way, because we are used to things being measured by the quantity, or weight of the material. For example, Merino Wool base layers are identified by the weight of the material; e.g. The 200 base layer means that the material weighs 200g per square meter. Fleece is measured in the same way, so is the insulation in winter boots. However down fill power has nothing to do with the amount of down in a winter coat. So what does fill power mean?!

Down, like any good insulation, works by trapping the warm air that is generated by your body. Assuming all things are equal, the more air that is trapped, the warmer you will be. Since not all birds are the same, it also means that not all down is the same either. Some birds (even from the same species) produce much fluffier down than others, so the measurement of fill power helps to indicate the quality of the fluffiness or loftiness of the down.

Photo: Allied Feather & Down

Fill power literally means: the amount of space (cubic inches) that 1oz of down occupies. This is tested by placing one ounce of down into a cylinder. Then a weight is placed on top and after one minute the volume of space that the down occupies is measured, and VOILA that is the fill power! So for example, 1 oz of 800 fill power down will occupy 800 cubic inches of space, and 1 oz of 650 fill power down will occupy 650 cubic inches of space.

The more lofty the down is, the more air it traps, and the warmer it will be…BUT WAIT! That doesn't mean that all jackets with 800 fill power will be warmer than all jackets with 650 fill power! Remember, fill power is the quality of the down, not the quantity. If all jackets only ever had 1 oz of down, then yes, an 800 fill jacket would be warmer than a 650 fill, because the 800 will be loftier, and trap more air. But every model of jacket is designed with different amounts of down. Jacket manufacturers often spec the highest quality down (700-900 fill) to save weight, and use it in pieces intended as a mid-layer which will be super light, but will not be as warm as a big parka with lots of 600 fill.

Think of it in terms of coffee - For the same price you can get a gourmet shot of espresso, or an XL Timmy’s. That little espresso packs a punch though, so both are going to give you the same caffeine kick. The same is true of down. The beauty of the higher fill power lies in the fact that they can provide the same warmth with less weight.

Remember: Down Fill Power is the quality, not the quantity of the down, and not the warmth of the jacket. There are also other factors that contribute to the warmth of a winter jacket, like the fabrics, features, length and whether it is wind and/or waterproof. Keep this in mind when shopping for a new jacket and you'll be well on your way to selecting the perfect one for your needs.

Here's a great video to show how down fill power is measured: